This invention relates to a sintered silicon nitride-silicon carbide composite material having a specific fine structure and a process for the production thereof. More specifically, it relates to a sintered silicon nitride-silicon carbide composite material having a composite structure consisting of a matrix phase and dispersion phase; the matrix phase has a silicon nitride-silicon carbide fine structure where silicon carbide grains having an average diameter of not more than 1 .mu.m are present at grain boundaries of silicon nitride grains and silicon carbide grains having a diameter of several nanometers to several hundred nanometers are dispersed within the silicon nitride grains, and the dispersion phase has a structure where (a) silicon carbide grains having an average diameter of 2 to 50 .mu.m and/or (b) silicon carbide whiskers having a short axis of 0.05 to 10 .mu.m and an aspect ratio of 5 to 300 are dispersed.
The present sintered silicon nitride-silicon carbide composite material has excellent fracture toughness, and also has excellent room-temperature and high-temperature strength. It is therefore useful as a high-temperature structural material, sliding material or wear-resistant material for a gas turbine, engine, and the like.
Silicon nitride and silicon carbide have recently been attracting special attention as an engineering ceramic for a high-temperature structural material. In particular, silicon nitride has excellent properties of outstanding heat and impact resistance and fracture toughness, and silicon carbide has excellent properties of outstanding oxidation resistance and high-temperature strength. For this reason, silicon nitride and silicon carbide are now under development utilizing these properties. Further, many attempts are also under way to develop silicon nitride-silicon carbide composite materials in order to utilize advantages of these two materials.
Silicon nitride-silicon carbide composite materials have been conventionally produced, e.g. by the following methods.
(1) A method of mixing a silicon nitride (Si.sub.3 N.sub.4) powder and a silicon carbide (SiC) powder or whiskers mechanically and sintering the mixture under pressureless or sintering it under pressure such as hot pressing, HIP, or the like.
(2) A method of using reaction sintering, in which silicon nitride (Si.sub.3 N.sub.4) is formed by a nitriding reaction of a shaped article formed of a silicon carbide (SiC) powder and a silicon (Si) powder, or in which silicon carbide (SiC) is formed by permeating silicon (Si) into a shaped article formed of a silicon nitride (Si.sub.3 N.sub.4) powder and carbon.
(3) A method of forming a silicon nitride-silicon carbide composite material by heating a mixture of an organo-silicon polymer or organo-silicon compound with a silicon (Si) powder.
Of these methods, the methods (2) and (3) advantageously exhibit good dimensional accuracy and excellent moldability. However, sintered bodies obtained in these two methods are liable to be porous, and it is difficult to produce a dense sintered composite material. Hence, the sintered bodies often have poor physical properties as compared to silicon nitride and silicon carbide. For example, the sintered bodies produced in these methods have lower strength than silicon nitride and silicon carbide.
In general, therefore, the above method (1) is usually employed to produce a dense sintered composite material. This method is generally classified into two groups; one is to add silicon carbide whiskers to silicon nitride, and the other is to add a silicon carbide powder to silicon nitride. Silicon nitride-silicon carbide composite materials in which whiskers are dispersed are described, e.g. in a YOGYO KYOKAI-SHI 91, 491 (1983) and 94, 981 (1986). That is, these authors describe that silicon nitride-silicon carbide composite materials which have high fracture toughness and low reduction in strength at high temperature and which have a large Weibull modulus for reliability can be obtained by mixing a silicon nitride powder with silicon carbide whiskers and sintering the mixture by hot-pressing.
On the other hand, examples of the silicon nitride-silicon carbide composite material using a silicon carbide powder are described, e.g. in U.S. Pat. No. 4,184,882, or J. Am. Ceram. Soc., 56,445 (1973). That is, these publications disclose that the addition of a silicon carbide (SiC) powder having a diameter of 5 to 32 .mu.m (40% by volume at maximum) to a silicon nitride (Si.sub.3 N.sub.4) powder can give a sintered composite material having improved thermal conductivity and high-temperature strength as compared with silicon nitride.
As described above, sintered composite materials produced by these conventional methods have partly-improved physical properties. However, these composite materials are not satisfactory for mechanical properties which have been recently required of heat-resistant materials for a gas turbine, etc. In view of the problem of this kind, the present inventors have already proposed in Japanese Patent Kokai (Laid-Open) No. 159256/1988 (corresponding to U.S. Pat. No. 4,800,182) that a silicon nitride-silicon carbide composite material, which is excellent in both room-temperature strength and fracture toughness over silicon nitride, can be obtained by homogeniously dispersing silicon carbide having an average diameter of not more than 1 .mu.m in silicon nitride to convert the silicon nitride to an elongated grains. Further, the present inventors have disclosed in Japanese Patent Application No. 31125/1989 that a sintered silicon nitride-silicon carbide composite material having a specific fine structure where silicon carbide is dispersed not only at grain boundaries of the silicon nitride but also within the individual silicon nitride grains is excellent in room-temperature/high-temperature strength, fracture toughness and heat insulation, and also has high hardness and excellent wear resistance. However, in order to make wide use of silicon nitride and silicon carbide which are brittle materials, it is still required to further improve their fracture toughness in particular.
It is an object of this invention to provide a sintered silicon nitride-silicon carbide composite material having excellent fracture toughness and excellent room-temperature and high-temperature strength over conventional silicon nitride-silicon carbide composite materials, and a process for the production thereof.